# Standard library header <ios>

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< cpp‎ | header

This header is part of the Input/Output library.

### Includes
<iosfwd> | Forward declarations of all classes in the input/output library
### Classes
ios_base | manages formatting flags and input/output exceptions (class)
basic_ios | manages an arbitrary stream buffer (class template)
`std::ios` | std::basic_ios<char> (typedef)
`std::wios` | std::basic_ios<wchar_t> (typedef)
fpos | represents absolute position in a stream or a file (class template)
io_errc (C++11) | the IO stream error codes (enum)
is_error_code_enum<std::io_errc> (C++11) | extends the type trait std::is_error_code_enum to identify iostream error codes (class template specialization)
streamoff | represents relative file/stream position (offset from fpos), sufficient to represent any file size (typedef)
streamsize | represents the number of characters transferred in an I/O operation or the size of an I/O buffer (typedef)
### Functions
iostream_category (C++11) | identifies the iostream error category (function)
make_error_code(std::io_errc) (C++11) | constructs an iostream error code (function)
make_error_condition(std::io_errc) (C++11) | constructs an iostream error condition (function)
boolalphanoboolalpha | switches between textual and numeric representation of booleans (function)
showbasenoshowbase | controls whether prefix is used to indicate numeric base (function)
showpointnoshowpoint | controls whether decimal point is always included in floating-point representation (function)
showposnoshowpos | controls whether the `+` sign used with non-negative numbers (function)
skipwsnoskipws | controls whether leading whitespace is skipped on input (function)
uppercasenouppercase | controls whether uppercase characters are used with some output formats (function)
unitbufnounitbuf | controls whether output is flushed after each operation (function)
internalleftright | sets the placement of fill characters (function)
dechexoct | changes the base used for integer I/O (function)
fixedscientifichexfloatdefaultfloat (C++11)(C++11) | changes formatting used for floating-point I/O (function)

### Synopsis

    #include <iosfwd>

    namespace std {
      using streamoff  = /* implementation-defined */;
      using streamsize = /* implementation-defined */;
      template<class StateT> class fpos;

      class ios_base;
      template<class CharT, class Traits = char_traits<CharT>>
        class basic_ios;

      // manipulators
      ios_base& boolalpha  (ios_base& str);
      ios_base& noboolalpha(ios_base& str);

      ios_base& showbase   (ios_base& str);
      ios_base& noshowbase (ios_base& str);

      ios_base& showpoint  (ios_base& str);
      ios_base& noshowpoint(ios_base& str);

      ios_base& showpos    (ios_base& str);
      ios_base& noshowpos  (ios_base& str);

      ios_base& skipws     (ios_base& str);
      ios_base& noskipws   (ios_base& str);

      ios_base& uppercase  (ios_base& str);
      ios_base& nouppercase(ios_base& str);

      ios_base& unitbuf    (ios_base& str);
      ios_base& nounitbuf  (ios_base& str);

      // adjustfield
      ios_base& internal   (ios_base& str);
      ios_base& left       (ios_base& str);
      ios_base& right      (ios_base& str);

      // basefield
      ios_base& dec        (ios_base& str);
      ios_base& hex        (ios_base& str);
      ios_base& oct        (ios_base& str);

      // floatfield
      ios_base& fixed      (ios_base& str);
      ios_base& scientific (ios_base& str);
      ios_base& hexfloat   (ios_base& str);
      ios_base& defaultfloat(ios_base& str);

      // error reporting
      enum class io_errc {
        stream = 1
      };

      template<> struct is_error_code_enum<io_errc> : public true_type { };
      error_code make_error_code(io_errc e) noexcept;
      error_condition make_error_condition(io_errc e) noexcept;
      const error_category& iostream_category() noexcept;
    }

#### Class std::ios_base

    namespace std {
      class ios_base {
      public:
        class failure;              // see description

        // fmtflags
        using fmtflags = /*bitmask-type-1*/;
        static constexpr fmtflags boolalpha = /* unspecified */;
        static constexpr fmtflags dec = /* unspecified */;
        static constexpr fmtflags fixed = /* unspecified */;
        static constexpr fmtflags hex = /* unspecified */;
        static constexpr fmtflags internal = /* unspecified */;
        static constexpr fmtflags left = /* unspecified */;
        static constexpr fmtflags oct = /* unspecified */;
        static constexpr fmtflags right = /* unspecified */;
        static constexpr fmtflags scientific = /* unspecified */;
        static constexpr fmtflags showbase = /* unspecified */;
        static constexpr fmtflags showpoint = /* unspecified */;
        static constexpr fmtflags showpos = /* unspecified */;
        static constexpr fmtflags skipws = /* unspecified */;
        static constexpr fmtflags unitbuf = /* unspecified */;
        static constexpr fmtflags uppercase = /* unspecified */;
        static constexpr fmtflags adjustfield = /* see description */;
        static constexpr fmtflags basefield = /* see description */;
        static constexpr fmtflags floatfield = /* see description */;

        // iostate
        using iostate = /*bitmask-type-2*/;
        static constexpr iostate badbit = /* unspecified */;
        static constexpr iostate eofbit = /* unspecified */;
        static constexpr iostate failbit = /* unspecified */;
        static constexpr iostate goodbit = /* see description */;

        // openmode
        using openmode = /*bitmask-type-3*/;
        static constexpr openmode app = /* unspecified */;
        static constexpr openmode ate = /* unspecified */;
        static constexpr openmode binary = /* unspecified */;
        static constexpr openmode in = /* unspecified */;
        static constexpr openmode out = /* unspecified */;
        static constexpr openmode trunc = /* unspecified */;
        static constexpr openmode noreplace = /* unspecified */

        // seekdir
        using seekdir = /*bitmask-type-4*/;
        static constexpr seekdir beg = /* unspecified */;
        static constexpr seekdir cur = /* unspecified */;
        static constexpr seekdir end = /* unspecified */;

        class Init;

        // fmtflags state
        fmtflags flags() const;
        fmtflags flags(fmtflags fmtfl);
        fmtflags setf(fmtflags fmtfl);
        fmtflags setf(fmtflags fmtfl, fmtflags mask);
        void unsetf(fmtflags mask);

        streamsize precision() const;
        streamsize precision(streamsize prec);
        streamsize width() const;
        streamsize width(streamsize wide);

        // locales
        locale imbue(const locale& loc);
        locale getloc() const;

        // storage
        static int xalloc();
        long&  iword(int idx);
        void*& pword(int idx);

        // destructor
        virtual ~ios_base();

        // callbacks
        enum event { erase_event, imbue_event, copyfmt_event };
        using event_callback = void (*)(event, ios_base&, int idx);
        void register_callback(event_callback fn, int idx);

        ios_base(const ios_base&) = delete;
        ios_base& operator=(const ios_base&) = delete;

        static bool sync_with_stdio(bool sync = true);

      protected:
        ios_base();

      private:
        static int index;           // exposition only
        long*  iarray;              // exposition only
        void** parray;              // exposition only
      };
    }

#### Class std::ios_base::failure

    namespace std {
      class ios_base::failure : public system_error {
      public:
        explicit failure(const string& msg, const error_code& ec = io_errc::stream);
        explicit failure(const char* msg, const error_code& ec = io_errc::stream);
      };
    }

#### Class std::ios_base::Init

    namespace std {
      class ios_base::Init {
      public:
        Init();
        Init(const Init&) = default;
        ~Init();
        Init& operator=(const Init&) = default;
      private:
        static int init_cnt;        // exposition only
      };
    }

#### Class template std::fpos

    namespace std {
      template<class StateT> class fpos {
      public:
        // members
        StateT state() const;
        void state(stateT);
      private;
        StateT st;                  // exposition only
      };
    }

#### Class template std::basic_ios

    namespace std {
      template<class CharT, class Traits = char_traits<CharT>>
      class basic_ios : public ios_base {
      public:
        using char_type   = CharT;
        using int_type    = typename Traits::int_type;
        using pos_type    = typename Traits::pos_type;
        using off_type    = typename Traits::off_type;
        using traits_type = Traits;

        // flags functions
        explicit operator bool() const;
        bool operator!() const;
        iostate rdstate() const;
        void clear(iostate state = goodbit);
        void setstate(iostate state);
        bool good() const;
        bool eof()  const;
        bool fail() const;
        bool bad()  const;

        iostate exceptions() const;
        void exceptions(iostate except);

        // constructor/destructor
        explicit basic_ios(basic_streambuf<CharT, Traits>* sb);
        virtual ~basic_ios();

        // members
        basic_ostream<CharT, Traits>* tie() const;
        basic_ostream<CharT, Traits>* tie(basic_ostream<CharT, Traits>* tiestr);

        basic_streambuf<CharT, Traits>* rdbuf() const;
        basic_streambuf<CharT, Traits>* rdbuf(basic_streambuf<CharT, Traits>* sb);

        basic_ios& copyfmt(const basic_ios& rhs);

        char_type fill() const;
        char_type fill(char_type ch);

        locale imbue(const locale& loc);

        char      narrow(char_type c, char dfault) const;
        char_type widen(char c) const;

        basic_ios(const basic_ios&) = delete;
        basic_ios& operator=(const basic_ios&) = delete;

      protected:
        basic_ios();
        void init(basic_streambuf<CharT, Traits>* sb);
        void move(basic_ios& rhs);
        void move(basic_ios&& rhs);
        void swap(basic_ios& rhs) noexcept;
        void set_rdbuf(basic_streambuf<CharT, Traits>* sb);
      };
    }

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 35 | C++98 | the prototypes of unitbuf and nounitbuf were missing in the synopsis | added
LWG 78 | C++98 | the type of parameter fn of `ios_base::register_callback` in the synopsis was misspecified as `event_call_back` | corrected to `event_callback`

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